Coated microfluidic devices and methods of making

ABSTRACT

We describe a method of layer-by-layer deposition of a plurality of layers of material onto the wall or walls of a channel of a microfluidic device, the method comprising: loading a tube with a series of segments of solution, a said segment of solution bearing a material to be deposited; coupling said tube to said microfluidic device; and injecting said segments of solution into said microfluidic device such that said segments of solution pass, in turn, through said channel depositing successive layers of material to perform said layer-by-layer deposition onto said wall or walls of said channel. Embodiments of the methods are particularly useful for automated surface modification of plastic, for example PDMS (Poly(dimethylsiloxane)), microchannels. We also describe methods and apparatus for forming double-emulsions.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is a National Stage of International ApplicationNo. PCT/GB2010/051092 filed on Jul. 2, 2010, which claims the benefit ofUnited Kingdom Patent Application No. 0911572.6 filed on Jul. 3, 2009.The entire disclosures of which are incorporated herein by reference.

FIELD OF THE INVENTION

This invention relates to methods for modification of the surfaces of achannel of a microfluidic device, to apparatus for performing suchmethods, and to microfluidic devices fabricated using or treated by suchmethods. Embodiments of the methods are particularly useful forautomated surface modification of plastic, for example PDMS(Poly(dimethylsiloxane)), microchannels.

BACKGROUND TO THE INVENTION

For several reasons poly(dimethylsiloxane) (PDMS) is one of the mostcommonly used materials in microfluidic chip fabrication. Compared tosilicon and glass devices, PDMS based chips can be manufactured muchfaster, easier and cheaper by means of soft lithography. Due to itselasticity pumps and valves can be introduced into PDMS devices.Furthermore, PDMS can be cured at low temperature, it is transparentdown to 280 nm, biologically inert and non-toxic as well as permeable togases. It also readily seals with other materials, such as glass andpoly(methyl methacrylate), which allows for the fabrication of hybridchips. However, significant limitations concerning the application ofPDMS in microfluidic devices arise from the high hydrophobicity of thematerial. For instance, the creation of oil-in-water emulsions insidemicrofluidic chips requires an effective wetting of the microchannelwalls with the continuous aqueous phase. Therefore, a surfacemodification is often necessary, although rather challenging because ofthe inertness of PDMS.

In literature numerous ways of PDMS surface modification can be found.One possible approach comprises the exposure of PDMS to various energysources, such as oxygen plasma. In this context, the generation ofhydrophilic surfaces by oxidation is only temporary though since PDMS isknown to regain its original hydrophobic surface properties over time, aphenomenon referred to as hydrophobic recovery.

Alternatively, chemical vapor deposition (CVD) can be used to introducepermanent coatings and hence adjust the surface properties of PDMS.However, since this method requires unhindered access of the vapor tothe substrate it is limited to the modification of non-assembledmicrofluidic chips. This is a significant drawback as the coating mustthen be stable enough to endure the bonding procedure typicallyinvolving plasma treatment.

Furthermore, PDMS surfaces can be modified covalently, most commonly viagraft photo-polymerization. A simple one-step strategy is availablewhich allows for the tailoring of PDMS surface properties by graftingvarious monomers. Again problems arise for assembled channels though,since polymerization predominantly occurs in the channel lumen ratherthan at the walls. The pre-adsorption of a suitable photo-initiatorsolves this problem, but requires additional preparation steps makingthe procedure more elaborate. Similarly, other covalent modificationstrategies, such as the generation of a glass coating via sol-gelmethods offer the possibility to modify PDMS permanently, but again in arather complex, labor-intensive and time-consuming manner.

Another surface modification method is based on the layer-by-layer (LbL)self-assembly of polyelectrolyte multilayers (PEMs) by alternateadsorption of polycations and polyanions. This versatile approach wasintroduced by the group of Decher [G. Decher, J. D. Hong, J. Schmitt,Thin Solid Films, 1992, 210/211, 831-835; G. Decher, Y. Lvov, J.Schmitt, Thin Solid Films, 1994, 244, 772-777; G. Decher, Science, 1997,277, 1232-1237].

Based on these results Katayama et al [H. Katayama, Y. Ishihama, N.Asakawa, Anal. Chem., 1998, 70, 2254-2260; H. Katayama, Y. Ishihama, N.Asakawa, Anal. Chem., 1998, 70, 5272-5277] developed an LbL procedureallowing for the treatment of capillary inner walls. Various othergroups like Barker et al, [S. L. R. Barker, M. J. Tarlov, H. Canavan, J.J. Hickman, L. E. Locascio, Anal. Chem., 2000, 72, 4899-4903; S. L. R.Barker, D. Ross, M. J. Tarlov, M. Gaitan, L. E. Locascio, Anal. Chem.,2000, 72, 5925-5929.] Henry et al [Y. Liu, J. C. Fanguy, J. M. Bledsoe,C. S. Henry, Anal. Chem., 2000, 72, 5939-5944] and Hahn et al. [K. W.Ro, W-J. Chang, H. Kim, Y.-M. Koo, J. H. Hahn, Electrophoresis, 2003,24, 3253-3259] are using similar protocols in order to modify surfaceproperties of microfluidic channel walls. However, all of these coatingprocedures rely on the manual flushing of a microfluidic chip withdifferent polyelectrolyte solutions. As additional washing steps need tobe carried out in between and all polyelectrolyte and washing solutionshave to be individually injected into the device, kept inside thechannels for some time and removed afterwards this method can easilybecome a tedious and labor-intensive task, too. Weitz et al. (A. R.Abate, D. A. Weitz, Small 2009, 5, 2030) have used photolithographictechniques to realize alternating wettability profiles insidemicrochannels for producing multiple emulsions.

Further background prior art can be found in U.S. Pat. No. 6,860,980,US2002/0053514, US2004/0084312, U.S. Pat. No. 6,402,918, andWO2005/052035.

There is therefore a need for improved techniques for surfacemodification of channels of a microfluidic device.

SUMMARY OF THE INVENTION

According to the present invention there is therefore provided a methodof layer-by-layer deposition of a plurality of layers of material ontothe walls of a channel of a microfluidic device, the method comprising:loading a tube with a series of segments of solution, a said segment ofsolution bearing a material to be deposited; coupling said tube to saidmicrofluidic device; and injecting said segments of solution into saidmicrofluidic device such that said segments of solution pass, in turn,through said channel depositing successive layers of material to performsaid layer-by-layer deposition onto said walls of said channel.

In preferred embodiments of the method the tube is loaded so that thereare gas, typically air, gaps between successive segments of solution, toact as spacers to thereby inhibit mixing of the solution in adjacentsegments. In embodiments segments of solution bearing material to bedeposited have segments of washing fluid between them, for example,water and/or an aqueous washing solution.

In embodiments the tube is loaded by coupling an end of the tube to eachof a set of solutions in turn in reverse order to that in which thesolutions are to be pumped through the microfluidic device (so that thetube need not be turned around once loaded). In embodiments of themethod the solution is injected at a controlled, for example, constantflow rate. In this way the duration of the deposition of a layer can becontrolled by controlling the flow rate and physical length of a segmentof solution bearing the material to be deposited.

In embodiments the channel of the microfluidic device is defined in aplastic, and the method further comprises pre-treating this plasticchannel by exposure to an energy source, for example a plasma, togenerate a hydrophilic surface for the channel. As previously mentionedthis hydrophilic surface can undergo hydrophobic recovery and therefore,in embodiments of the method, the layer-by-layer deposition is commencedbefore this hydrophobic recovery has proceeded more than 10%, 20%, 30%,40% or 50%.

In embodiments the plastic may comprise PDMS, but embodiments of themethod are also suitable for many other plastic/polymer materials,including but not limited to: polystyrene, PETG (Poly(ethyleneterephthalate glycol)), PMMA (poly(methacrylate)) and polycarbonate.

In preferred embodiments of the method the material to be depositedcomprises polyelectrolyte. The skilled person will be aware of manysuitable materials in some preferred implementations of the method thepolyelectrolyte solutions comprise solutions of positively andnegatively charged polyelectrolytes, for example PAH and PSSrespectfully. In this way alternating positively and negatively chargedpolyelectrolyte layers may be deposited on the walls of the microchannelto provide a multilayer coating. In embodiments the segments ofpolyelectrolyte are separated by aqueous washing solution and a gas(air) gap at either or both ends. The skilled person will be aware,however, that embodiments of the above described technique may beemployed to deposit a layer or layers of many other different materials.

The invention also provides a method of fabricating a microfluidicdevice including treating one or more channels of the device asdescribed above. The invention further provides a microfluidic devicefabricated using or treated by a method as described above.

Embodiments of the above described method are particularly advantageousfor treating the device for processing droplets of an emulsion in anaqueous stream of fluid, such as an oil-in water emulsion.

The invention also provides apparatus for layer-by-layer deposition of aplurality of layers of material onto the walls of a channel of amicrofluidic device, the apparatus comprising: a system for loading atube with a series of segments of solution, a said segment of solutionbearing a material to be deposited; and a system for injecting saidsegments of solution into said microfluidic device such that saidsegments of solution pass, in turn, through said channel depositingsuccessive layers of material to perform said layer-by-layer depositiononto said walls of said channel.

In embodiments the system for injecting the solution segments into themicrofluidic device may comprise a controlled-rate pump, for example, asyringe pump; the same or a similar arrangement may be employed forloading the tube with the segments of solution prior to injection. Inembodiments the tubing comprises a length of replaceable flexibletubing, for example of plastic, which may conveniently be cut to removeunwanted contamination. Additionally or alternatively a fluid switch ormultiplexer may be employed to selectively couple the tubing to aplurality of reservoirs bearing solutions to be loaded into the tubeand/or to selectively couple the tube to a microfluidic device to betreated.

As described further below, embodiments of these techniques may also beemployed to provided microfluidic devices suitable for the production ofdouble emulsions, in particular (water in oil) in water emulsions. Thusembodiments of the above described methods may comprise flowing thesegments of solution alongside a second fluid such that the layers ofmaterial from the layer-by-layer deposition process are selectivelydeposited on a lateral portion of the channel. More particularly the twofluids/flows are chosen or configured such that the two fluids flowalongside one another without substantial mixing and in a relativelystable joint flow pattern so that one longitudinal side of the channel(one or more walls or a portion of a curved side wall) is selectivelycoated. Depending on the second fluid flow the remaining longitudinalside portion of the channel may or may not be coated or otherwisemodified.

Thus in a further aspect the invention provides a method of providing asurface modification pattern on the wall or walls of at least onechannel of a microfluidic device, the method comprising: flowing firstand second fluids through said channel of said microfluidic device suchthat within said channel said first and second fluids remainsubstantially unmixed with one another and define a substantially stablejoint fluid flow pattern; and providing at least one of said fluid flowswith a surface modification agent such that said wall or walls of saidchannel are modified by said surface modification agent to acquire asurface modification pattern defined by said joint fluid flow pattern.

In embodiments the surface modification pattern may comprisemodification of a surface energy of the channel, for particularly tomake a portion of the side wall or walls of the channel hydrophilic orhydrophobic. In embodiments this may be achieved by selectivelydepositing material on one or another lateral side of the channel, inparticular layer-by-layer as described above, but in other approachesother techniques for surface energy modification may be employed.Although two fluids are employed in some preferred embodiments, inprinciple flows of more than two fluids which form a relatively stableflow pattern and remain substantially mutually separate from one anothermay be used.

In some preferred implementations of the method the first and secondfluids are provided in respective first and second channels whichprovide inlets to a fluid flow junction from which one, two or morethird channels provide an outlet or outlets. Surface modificationpatterning takes place in the joint fluid flow in the third channel(s),so that one side of a third channel is modified and another side, forexample the opposite side, of the third channel remains substantiallyunmodified or is modified in a different way. As previously mentionedthe surface modification may comprise providing the surface with ahydrophilic or hydrophobic surface modification, for example a surfacecoating, in particular a layer-by-layer coating using a method asdescribed above.

The two fluid flows may both comprise liquid fluid flows or one of thefluids may comprise a gas (which here includes air). The fluids shouldbe substantially immiscible in the microfluidic channel, but this doesnot necessarily mean that they are immiscible under all conditions—forexample the surface energy of one or both of the fluids may mean thatwithin the narrow confines of the channel it is energeticallyunfavourable for the fluids to mix.

In embodiments the method is performed in a microfluidic deviceconfigured to form a double emulsion. Thus a combination of a saidfirst, second and third channel and junction may form a flow focussingregion for forming a droplet of the emulsion, and a pair of such flowfocussing regions may be employed, one downstream of the other, to forma double emulsion. In this way, for example, the first flow focussingregion may be employed to form a droplet of water in oil and the secondflow focussing region to form a double emulsion comprising a droplet ofoil in water wherein the oil itself comprises one (or more) waterdroplets.

Thus in a related aspect the invention provides a microfluidic devicehaving at least, first, second and third fluid flow channels, said firstand second channels providing inlets to a fluid flow junction and saidthird channel providing an outlet from said fluid flow junction, andwherein said third channel has a surface modification pattern such thata first longitudinal side of said third channel is more hydrophilic thana second, opposite longitudinal side of said third channel and saidsecond longitudinal side of said third channel is more hydrophobic saidfirst longitudinal side of said third channel.

In embodiments the device comprises pair of flow focussing regions asdescribed above, one downstream of the other, such that the device isconfigured to form a double emulsion.

The invention also provides a method of forming a double emulsion usingsuch a device. In embodiments of the method the device is controlled ordriven such that the droplet formation at the first and second flowfocussing regions is substantially in synchronism, for example byarranging for the droplet formation in these two regions to proceed atsubstantially the same rate.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other aspects of the invention will now be further described,by way of example only, with reference to the accompanying figures inwhich:

FIGS. 1 a and 1 b show, respectively, a schematic diagram illustrating amethod and apparatus according to an embodiment of the invention, and across section through a treated wall of a microfluidic channel;

FIGS. 2 a and 2 c shows fluorescence analysis of automated LbL(layer-by-layer) PDMS surface modification showing, respectively, (a)schematics of flushing sequences with varying number n of fluorescentlylabelled PAH segments, (b) fluorescence microscopic image of fourstraight microfluidic channels (fluorescence intensity increases withincreasing n), (c) fluorescence microscopic analysis of a wiggle channelreveals a homogeneous coating even for this geometry;

FIGS. 3 a to 3 d show the influence of PDMS surface modification on theformation and stability of oil-in-water droplets, scale bar=100 μm,respectively (a), (b) without further treatment of the channelsinstabilities and phase reversion occur; (c), (d), after LbL depositionof a PEM—the PDMS surface maintains its hydrophilicity even after 2.5weeks of storage under air allowing for the formation of monodisperseand stable droplets;

FIG. 4 shows selective hydrophilic surface coating of certain parts ofthe microchannel showing (a) a schematic illustration of a microfluidicdevice used for the generation of water/oil/water double emulsions (forhydrophilic surface treatment a polyelectrolyte sequence was flushedthrough inlet D, while deionised water (DIW) was injected through inletB; inlet C was blocked and A was used as an outlet); (b and c) showbright field micrographs of the microfluidic device after hydrophilicsurface treatment; and (d and e) show corresponding fluorescencemicrographs of the microfluidic device after hydrophilic surfacetreatment (only the lower part of the microchannel was coated with afluorescent PEM; no deposition occurred within the upper part; scalebars denote (a) 2 mm, (b) 750 μm; (c) 150 μm, (d) 750 μm and (e) 150μm); and

FIG. 5 shows production of a water/oil/water (w/o/w) double emulsionusing a partially hydrophilic, partially hydrophobic device showing (aand b) micrographs of stable enclosing of small water droplets in thebigger oil droplets, (c) a micrograph depicting a hexagonal close-packedmonolayer form adopted by the w/o/w emulsion in a storage device, and(d) a diameter distribution showing a high level of dropletmonodispersity both for the inner and the outer droplets (scale barsdenote (a) 500 μm; (b) 200 μm and (c) 200 μm).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

We will describe a LbL approach which provides time-saving, automatedsurface modification process (FIG. 1) and which allows for the creationof stable oil-in-water droplets (in droplet-based fluidics, the presenceof biphasic flow puts great demands on the uniformity and surface energyof the channel coating). FIG. 1 shows a schematic illustration of theautomated LbL surface modification of a microfluidic channel 102 of amicrofluidic chip 100. In FIG. 1 a defined segments of aqueous solutionsof NaCl, a positively and a negatively charged polyelectrolyte (PAH andPSS, respectively), separated by air and stored within the tubing, aresequentially flushed through the microfluidic channel at a constant flowrate. In FIG. 1 b at the PDMS surface a coating of alternating positiveand negative polyelectrolyte layers is successively built up.

A key feature is the loading of a piece of tubing 10 with definedsolution segments 14, separated by air. In this regard, one end of thetubing is attached to a syringe 12 and the other end is dipped into thesolution which shall enter the channel last later on. Having withdrawn asolution segment of a certain length into the tubing the latter ispulled out of the solution. An air segment 16 is drawn in and thecontaminated part of the tubing which was in contact with the solutionis simply cut off. The next solution segment can be sucked in likewiseand so on. In order to build up a polyelectrolyte multilayer 106 we loadthe tubing alternately with segments of poly(allylamine hydrochloride)(PAH) and poly(sodium styrene sulfonate) (PSS) solutions (1 mgpolyelectrolyte in 1 ml 0.5 M aqueous NaCl solution in both cases) with0.1 M aqueous NaCl washing solution segments in between. But the conceptcan of course be applied for all imaginable combinations of solutions.The loading of the tubing with the desired number of segments is theonly task within the procedure which is, in embodiments, carried outmanually but usually takes no longer than 10 minutes (in otherimplementations this may be automated). Subsequently, the tubing isconnected to the microfluidic chip 100 directly after plasma treatmentand assembly, when the channel walls 104 are still hydrophilic. Using asyringe pump 12 the segments successively enter the chip, flush it andare blown out again by the respective following air segment in acompletely automatic fashion. Applying a constant flow rate the lengthof the individual segment precisely determines the time thecorresponding solution stays inside the microchannel. The final washingstep is performed with distilled water in order to remove traces of saltfrom the channel walls.

We were already able to show the successful operation of this concept(FIG. 2) Within a fluorescence study we systematically varied the numbern of fluorescently labeled PAH segments within the tubing FIG. 2 a). Theanalysis of the microchannels under the fluorescence microscope revealedan increase in fluorescence intensity with increasing numbers of labeledsegments FIG. 2 b). This shows that we are able to build up PEMs ontothe channel walls in a controlled manner. Our automated LbL approach isnot limited to straight channels but also allows for a homogeneouscoating of more complex geometries FIG. 2 c).

Furthermore, we studied extensively the effects of channel surfaceproperties on the formation and stability of oil-in-water droplets.Referring to FIG. 3, when the PDMS is not further modified after plasmatreatment the hydrophobic recovery leads to instabilities and even phasereversion within no more than one hour, even if the experiment isstarted immediately after oxidation and chip assembly FIGS. (3 a), (3b). In contrast, when depositing a PEM by our automated procedure thehydrophilicity is preserved and stable, monodisperse oil-in-waterdroplets can be formed even after storing the device under air for atleast 2.5 weeks after modification FIGS. (3 c), (3 d).

Since our automated surface modification method deals with a fundamentalissue in microfluidics, i.e. the fast creation of hydrophilic PDMSchannels with long-term stability, the possible benefits aresubstantial. Whenever well-defined oil-in-water droplet are to begenerated in microfluidic devices, e.g. for organic synthesis or for thecreation of nano- and microparticles, our approach is a potentiallyuseful option. Being not restricted to surface modification based onpolyelectrolyte solutions and allowing for the selective modification ofcertain channels within one chip our technique also opens up new avenuesfor applications beyond the examples described above. Broadly speakingwe have described a new automated technique for the modification of PDMSmicrochannels. Embodiments of the method may be employed with channelsof a range of sizes, but are preferably employed with (microfluidic)channels having a maximum transverse dimension of less than 1 mm (oftenless than 500 μm or 300 μm), and may be employed with much smallerchannels, for example less than 1 μm maximum transverse dimension.Embodiments of the methods combine a convenient and time-saving processon the assembled chip with versatility and long-term channelhydrophilicity. Microfluidic devices are configured for laminar flow andtypically have a Reynolds number of <30, <10, <2 or <1.

Applications of embodiments of the invention are not limited to chipsand may be employed with other microfluidic systems including, but notlimited to: inkjet print heads, microarrays and otherchemical/biochemical sensors and, more generally, to a range ofmicrofluidic systems for handling fluids. Embodiments are, however,particularly advantageous for on-chip microfluidic structures such asso-called lab-on-a-chip (LOC) devices.

Surface Patterning Via Selective LbL Deposition for the Production ofDouble Emulsions

Having previously described our LbL deposition technique for theproduction of oil/water (o/w) microdroplets we now address thefabrication of more complex devices that can generate double emulsions.Especially water/oil/water double emulsions are highly attractivesystems as they have great potential for applications, for instance asdelivery systems for pharmacological compounds. As bulk preparationmethods usually yield water/oil/water droplets with a broad sizedistribution a microfluidic approach is advantageous when monodispersedouble emulsions are required. This is not trivial as two dropletproduction processes have to be precisely controlled. In order to engulfa defined number of small water droplets into the bigger oil dropletsboth formation frequencies should be synchronized. This is done bestwhen performing both processes on a single microfluidic chip. However,when using PDMS-based devices the wettability of the channel surfaceposes another challenge. For the formation of the inner water dropletsthe channel wall should be hydrophobic whereas for the production of theouter oil droplets hydrophilic surface properties are desirable. Hence,the surface energy of the microchannel should be patterned.

We have used the above described LbL deposition technique to selectivelyapply a hydrophilic channel coating in certain areas of the microfluidicdevice (FIG. 4). In FIG. 4( a) a schematic diagram of a microfluidicdevice (chip) 400 capable of forming double emulsions is shown. Themicrochannel device comprises a narrow channel part 402 (upper part; 50μm wide channels) with a first flow focusing region (junction) 404 wherechannels 406 a,b; 408 from inlets B and C meet and having an outletchannel 410; and a wider channel part 412 (lower part; 200 μm widechannels). These sections meet at a second flow-focusing region(junction) 414 at which the outlet channel 410 from the first flowfocusing region and (in the illustrated example) a pair of channels 418a,b from inlet A meet. An outlet channel 416 from the secondflow-focusing region (junction) to outlet D is provided. At the secondflow-focusing region 414 the incorporation into, preferably theenclosure of, the small water droplets into larger oil droplets, andhence wettability reversal, should occur. In order to produce stablew/o/w double emulsions the wider (lower) part should be turnedhydrophilic selectively over the narrower (upper) one. Directly afterdevice assembly we injected a polyelectrolyte sequence containing FITC(fluorescein isothiocyanate) labeled PAH in inlet D and flushed itthrough the lower part of the chip at a constant flow rate of 50 μL h⁻¹.Simultaneously we blocked the upper part of the device by streaming DIWthrough inlet B at 100 μL h⁻¹. Inlet C was closed during themodification process while A was used as an outlet.

By means of fluorescence microscopy we analyzed the microfluidic deviceafter this hydrophilic treatment. FIG. 4( b) and FIG. 4( c) show brightfield micrographs of the area around the second flow-focusing region.The corresponding fluorescence micrographs in FIG. 4( d) and FIG. 4( e)reveal that only in the designated lower part of the microchannel afluorescent PAH-PSS-PAH-PSS PEM was deposited. In contrast, the absenceof a fluorescence signal in the upper part of the chip shows that no LbLcoating took place there. The boundary between the modified and thenon-modified channel section can be located at the second flow-focusingregion. These results demonstrate that following our LbL surfacemodification method we are able to partially coat the microchannel wallwith a hydrophilic PEM in a precise and spatially defined manner. Nolithographic tools are needed, a flow pattern is transferred instead.The partial surface modification technique we describe is very robustfor the geometries tested here, and could be extended to morecomplicated channel designs, e.g. chips with multiple flow-focusingjunctions.

In a second surface modification step we produced a defined hydrophobicsurface in the upper part of the microchannel. Again we used flowpatterning to achieve a partial coating. In a reversed setup compared tothe hydrophilic modification step we slowly injected the commercialwater repellent agent Aquapel through inlet B at 20 μL h⁻¹. A stream ofair was blown in the channel through inlet D at 5000 μL h⁻¹ to inhibitAquapel from interfering with the PEM deposited in the lower part of thechip. While inlet C was blocked during this process A was used as anoutlet.

Subsequently, we used the resultant partially hydrophilic, partiallyhydrophobic device for the production of a w/o/w double emulsion (FIG.5). DIW (deionised water) containing 0.5% w/w SDS (sodium dodecylsulphate) was injected as the outermost phase through inlet A at 1500 μLh⁻¹. Through inlet B we flushed the fluorous oil FC-40 containing 2.0%w/w EA surfactant (C. Holtze, A. C. Rowat, J. J. Agresti, J. B.Hutchison, F. E. Angilè, C. H. J. Schmitz, S. Köster, H. Duan, K. J.Humphry, R. A. Scanga, J. S. Johnson, D. Pisignano and D. A. Weitz, LabChip 2008, 8, 1632) at a flow rate of 200 μL h⁻¹. Pure DIW was used asthe innermost phase and applied through inlet C at 150 μL h⁻¹. In FIG.5( a) and (b) the formation of the double emulsion is depicted. DIW inoil droplets formed at the first flow-focusing region travel smoothlythrough the upper part of the channel. The microchannel wall is wettedby the fluorous oil indicating that the Aquapel treatment wassuccessful. At the second flow-focusing region every DIW droplet isenclosed in a larger oil droplet. Substantially no oil droplet staysempty since both droplet formation steps proceed at the same frequency(in an example, of 130 Hz). The double emulsion droplets proceed throughthe lower part of the microchannel without any wetting problems. Duringthe whole observation period of 90 min no signs of degradation inhydrophilic surface properties occurred.

The resultant double emulsion was extruded through outlet D andcollected for 30 min. A small fraction of ca. 5 μL was injected into astorage device. FIG. 5( c) shows a micrograph of the hexagonalclose-packed monolayer adopted by the double emulsion droplets insidethe reservoir. The droplet diameter distribution of 1107 droplets, shownin FIG. 5( d), is extremely narrow for both the inner water and theouter oil droplets and yields mean values of 84.6 μm and 109.4 μm,respectively. The standard deviations, 0.8 μm for the inner and 1.0 μmfor the outer droplets, are below 1% of the corresponding averagediameter, showing the high level of monodispersity in both cases.

In this experimental study we showed that our LbL surface depositiontechnique is not limited to simply turning a complete microchannelhydrophilic: It is also possible to precisely pattern the surface energywithin an assembled channel or channels. One application is thefabrication of semi-hydrophilic, semi-hydrophobic microfluidic devicescapable of producing highly monodisperse w/o/w double emulsions.

No doubt many other effective alternatives will occur to the skilledperson. It will be understood that the invention is not limited to thedescribed embodiments and encompasses modifications apparent to thoseskilled in the art lying within the spirit and scope of the claimsappended hereto.

The invention claimed is:
 1. A method of layer-by-layer deposition of aplurality of layers of material onto the wall or walls of a channel of amicrofluidic device, the method comprising: loading a tube with a seriesof segments of solution, a said segment of solution bearing a materialto be deposited; coupling said tube to said microfluidic device; andinjecting said segments of solution into said microfluidic device suchthat said segments of solution pass, in turn, through said channeldepositing successive layers of material to perform said layer-by-layerdeposition onto said wall or walls of said channel.
 2. A method asclaimed in claim 1 wherein said loading further comprises loading saidtube with gas between said segments of solution to separate saidsegments of solution.
 3. A method as claimed in claim 1 wherein saidloading further comprises loading said tube with a segment of washingfluid between two of said segments of solution bearing material to bedeposited.
 4. A method as claimed in claim 1 wherein said loadingcomprises coupling an end of said tube to each of a set of saidsolutions in turn in a reverse order to an order in which materials insaid solutions are to be deposited and withdrawing said segments of saidsolution into said tube.
 5. A method as claimed in claim 1 wherein saidinjecting comprises injecting at a controlled rate, and furthercomprising controlling a duration of deposition of a said layer using acombination of said controlled rate and a physical length of a saidsegment in said tube.
 6. A method as claimed in claim 1 wherein saidchannel of said microfluidic device is defined in a plastic, the methodfurther comprising pre-treating said plastic channel by exposure to anenergy source to generate a hydrophilic surface on said wall or walls ofsaid channel, wherein said hydrophilic surface undergoes hydrophobicrecovery, and wherein of said layer-by-layer deposition of saidplurality of layers of material is started prior to 50% hydrophobicrecovery of said hydrophobic surface.
 7. A method as claimed in claim 1wherein said loading of said tube comprises loading said tube withsuccessive said segments of solution comprising, alternately, positivelyand negatively charged polyelectrolyte each separated by washingsolution and at least one gap segment from which fluid is substantiallyabsent.
 8. A method as claimed in claim 1 further comprising flowingsaid segments of solution alongside a flow of a second fluid in saidchannel such that said plurality of layers of material are selectivelydeposited on a lateral portion of said channel.
 9. A method of using amicrofluidic device to process droplets of an emulsion in an aqueousstream of fluid, the method comprising depositing a plurality of layersof material onto the wall or walls of a channel of the microfluidicdevice using the method of claim 1 to render said wall or wallshydrophilic, and processing said droplets of said emulsion by passingsaid aqueous stream through said channel of said microfluidic device.10. A method of fabricating a microfluidic device comprising treatingthe microfluidic device using the method of claim
 1. 11. A method ofproviding a surface modification pattern on the wall or walls of, atleast one channel of a microfluidic device, the method comprising:flowing first and second fluids through said channel of saidmicrofluidic device such that within said channel said first and secondfluids remain substantially unmixed with one another and define asubstantially stable joint fluid flow pattern; and providing at leastone of said fluid flows with a surface modification agent such that saidwall or walls of said channel are modified by said surface modificationagent to acquire a surface modification pattern defined by said jointfluid flow pattern, wherein said flowing comprises flowing said firstfluid along a first channel towards a fluid flow junction and flowingsaid second fluid along a second channel towards said fluid flowjunction and wherein said joint fluid flow pattern comprises a jointflow of said first and second fluids away from said junction in a thirdchannel, wherein said first and second fluids flow alongside one anotherin said third channel such that said wall or walls on one longitudinalside of said third channel are provided with said surface modificationpattern.
 12. A method as claimed in claim 11 wherein said microfluidicdevice comprises a device for forming a double emulsion having first andsecond flow-focussing regions, each with a respective said first, secondand third channel and junction, and wherein the method comprisesproviding said surface modification pattern in said third channel of oneor both of said first and second flow-focussing regions.
 13. A method asclaimed in claim 11 wherein said surface modification pattern comprisesa surface coating pattern, and wherein said fluid flow provided withsaid surface modification agent comprises a fluid flow bearing amaterial to perform layer-by-layer deposition of said surface coatingpattern.